(a) Technical Field
The present invention relates to a hydrogen concentration control device and method for a fuel cell system. More particularly, the present invention relates to a hydrogen concentration control device and method for a fuel cell system, which are adapted to maintain the hydrogen concentration of an anode in the fuel cell system at a proper level in accordance with the concentration of hydrogen supplied to the fuel cell of the fuel cell system.
(b) Background Art
A fuel cell system typically includes a hydrogen supply system and an oxygen supply system for supplying reactant gases (hydrogen and oxygen in the air); a fuel cell stack for electrochemically converting chemical energy derived from an oxidation and reduction reaction of the reactant gases into electric energy to generate heat as a product of the reaction, and electric energy; and a water and heat management system for cooling the fuel cell stack.
Among these components of the fuel cell system, the fuel cell stack for practically generating electricity has a stacked structure of tens to hundreds of unit cells, each of which is comprised of a membrane electrode assembly (MEA) (which may also be referred to as an electrode-membrane assembly or an electrode-membrane joint body), and a separator, in which a pair of end plates are mounted on the opposite ends of the fuel cell stack to fix the individual stacked components together with a predetermined face pressure as well as to conduct electricity collection.
The MEA includes a polymer electrolyte membrane, an anode and a cathode which are arranged with the polymer electrolyte membrane interposed therebetween, in which the anode (which may also be referred to as a hydrogen electrode, a fuel electrode, a negative-pole electrode, or an oxidation electrode) and the cathode (which may also be referred to as an air electrode; an oxygen electrode, a positive-pole electrode, or a reduction electrode) are fabricated by adsorbing a catalytic layer containing nano-sized particles of catalysts onto a backing layer.
Now, the electricity generation principle of such a fuel cell system will be briefly described. As hydrogen is supplied to the anode of the fuel cell stack, an oxidation reaction of the hydrogen is initiated on the anode, thereby producing hydrogen ions (protons) and electrons. The hydrogen ions and electrons produced thereby are moved to the cathode through an electrolyte membrane and a separator. Then, water is produced on the cathode through an electrochemical reaction between the hydrogen ions and electrons moved from the anode and oxygen in the air, and the electric energy finally generated from the currency of such electrons is supplied to a load (e.g., a motor for driving a fuel cell vehicle), that requires the electric energy, through an electricity collector of the end plates.
Meanwhile, a conventional hydrogen concentration control technology for maintaining the hydrogen concentration of the anode of a fuel cell stack at a proper level by measuring the concentration of hydrogen supplied to the anode has been applied to various conventional hydrogen supply systems for supplying hydrogen using a modifier. For example, the conventional control technology confirms and controls the concentration of hydrogen produced through such a modifier and the content of impurities in the hydrogen so that hydrogen is supplied with a concentration equal to or higher than a reference concentration. If impurities harmful to the catalytic layer are contained in the hydrogen, the driving of the hydrogen supply system is stopped.
In addition, another conventional hydrogen concentration control technology has been applied to a hydrogen supply system. The conventional hydrogen concentration control technology measures the impedance of a fuel cell stack to determine the hydrogen concentration in the anode channel of the fuel cell on the basis of the measured impedance. When the hydrogen concentration is lower than a reference value, the conventional hydrogen control technology maintains the hydrogen concentration through the purge control of the anode. However, there is a difficulty in precisely measuring the hydrogen concentration by measuring the impedance from a fuel cell vehicle, of which the fuel cell practically suffers from continuous variation in current load. In particular, there is a problem in that the fuel cell should be necessarily equipped with an expensive impedance measuring device.
Still another conventional technology has been applied, in which the conventional technology provides a hydrogen concentration measuring sensor in either a hydrogen recirculation tube or at an outlet end of an anode of a fuel cell stack to confirm the concentrations of hydrogen and impurities of the anode. This technology confirms the power state of each of unit cells of the fuel cell stack. When the power is lower than a reference value due to excessive impurities in the corresponding unit cell, the conventional technology performs hydrogen purge to remove the impurities.
In this embodiment, however, when the hydrogen concentration is confirmed in the recirculation tube side, there is a difficulty in practically applying the conventional technology because the recirculated quantity of hydrogen is not uniform in accordance with the operating conditions of the fuel cell, and it is very difficult to precisely measure the hydrogen concentration under the influence of condensation water. Furthermore, there is a sufficiently high possibility that damage and durability reduction may still occur to the anode in this technology because the hydrogen purge process is often performed after the problem has already occurred. In addition, because a large quantity of condensation water actually exists at the outlet end of the fuel cell anode, which makes it difficult for the hydrogen concentration measuring sensor to normally operate at the outlet end, and because the probability of measurement error and breakdown of the sensor caused due to the water is very high, it is hard to apply this conventional technology to an actual product.
Yet another conventional technology has been applied, in which the conventional technology provides a hydrogen concentration measuring sensor in a hydrogen recirculation tube to confirm the concentration of impurities in an anode channel, and then performs hydrogen purge control on the basis of this confirmation. However, since a large quantity of condensation water exists in the recirculation tube, it is difficult to measure the hydrogen concentration under most circumstances. In addition, again it is very likely that the above process will be performed after an electrode of the fuel cell has already been damaged due to the impurities in the anode, and thus as a result the durability of anode is again greatly effected as a result.